The high cost of optical elements applicable to the infrared band remains an ongoing challenge. The cost of optical elements is determined in part by the price of constituents composing such elements, processing methods required to form the constituents into ingots, and fabrication processes required to properly manufacture precision optical elements from ingots. For example, infrared lenses are typically fabricated from single-crystal germanium, a costly elemental material. Single-crystal forms of germanium are produced via costly growth processes. Furthermore, single-crystal germanium is shaped to form optical lenses via cumbersome and costly mechanical methods, including cutting, grinding, polishing, and edging, the latter ensuring both optical and mechanical axes are properly aligned. Complex lens designs require even more expensive manufacturing methods, one example being the single-point-diamond method.
Chalcogenide glass compositions are broadly described as amorphous systems composed of one or more group VI elements, examples including sulfur (S), selenium (Se), and tellurium (Te), and one or more group III, IV, and/or V elements, examples including arsenic (As), germanium (Ge), antimony (Sb), tin (Sn), and gallium (Ga), having applicability to infrared optical elements. Compositions have low characteristic vibration frequencies allowing transmission far into the infrared region and band passes from the visible to 15 microns. Chalcogenide compositions may be melt processed to form glass ingots rather than grown to form costly crystals. As such, chalcogenide compositions facilitate the manufacture of netshape long wavelength infrared (LWIR) lenses via melt processes rather than mechanical methods. Netshape manufacturing methods offer significant economic and ecological advantages over multi-step mechanical processes because of the elimination of intermediate manufacturing steps and waste materials.
Several chalcogenide glass compositions have been developed for use as infrared optical components fabricated via hot pressing processes; however, several significant deficiencies are inherent to such compositions. Most compositions are composed of one or more costly elements, including germanium. Compositions have a relatively high glass softening temperature requiring processing at elevated temperatures, typically 320 to 500 degrees Celsius, which further increases manufacturing costs and difficulty. Compositions have a thermal expansion coefficient which differs from molds used in hot pressing processes, thus causing mismatch between lens surface and mold during cool down resulting in an improperly shaped lens. Compositions have a thermal coefficient of refractive index on the order of 10−5/° C., which is indicative of temperature sensitive performance.
Another problem within the related arts includes the limited design options afforded by germanium-free compositions. For example, Hilton, in U.S. Pat. No. 7,157,390, describes and claims a composition composed of arsenic and selenium with a 4% range of variation on a weight basis for each of the two constituents. Optical designs in general would benefit from the development of a variety of infrared glasses with different refractive indexes while maintaining desired thermal expansion coefficient and thermal coefficient of refractive index values.
Presently, the problems related to the mismatch between the coefficients of thermal expansion for known chalcogenide glass compositions and hot pressing molds are extremely difficult and costly to solve. The shrinkage of a lens within a mold is a very complicated process with no practical theory to estimate the effect. As such, an iterative process is applied including the steps of fabricating a mold, manufacturing a lens with the mold, measuring the resultant lens to determine its deviation with the required design, and redesigning the mold to compensate for observed deviations. Typically, the process requires numerous iterations before a mold yields the desired lens. Accordingly, the problems described above are minimized when the coefficient of thermal expansion for both glass composition and mold are comparable and avoided when they are identical.
It may be appreciated, therefore, that there remains a need for further advancements and improvements thus enabling the manufacture of thermally-stable, infrared optical elements.
Accordingly, what is required is a low-cost, germanium-free chalcogenide glass composition having a thermal expansion coefficient compatible with molds commonly used with hot pressing processes, a temperature coefficient of refractive index which varies little with temperature and otherwise referred to herein as near zero, a low glass transition temperature, and a low glass softening temperature.
What is also required are chalcogenide glass compositions which provide a broader range of design options so as to achieve the desired optical performance for an optical design while maintaining a thermal expansion coefficient of 23.6×10−6/° C. and a thermal coefficient of refractive index less than 1×10−6/° C.